DEV: Call Discourse.redis.flushdb after the end of each test (#29117)

There have been too many flaky tests as a result of leaking state in
Redis so it is easier to resolve them by ensuring we flush Redis'
database.

Locally on my machine, calling `Discourse.redis.flushdb` takes around
0.1ms which means this change will have very little impact on test
runtimes.

spec/lib/site_settings/deprecated_settings_spec.rb

@@ -34,10 +34,6 @@ RSpec.xdescribe SiteSettings::DeprecatedSettings do
   describe "when not overriding deprecated settings" do
     let(:override) { false }
 
-    # Necessary because Discourse.deprecate uses redis to see if the warning
-    # was already logged.
-    use_redis_snapshotting
-
     # NOTE: This fixture has some completely made up settings (e.g. min_trust_level_to_allow_invite_tl_and_staff)
     let(:deprecated_test) { "#{Rails.root}/spec/fixtures/site_settings/deprecated_test.yml" }